1. Field of the Invention
Exemplary embodiments of the present invention relate to a band-pass filter and a duplexer using the same; and, more particularly, to a band-pass filter based on a CRLH (Composite Right/Left-Handed) resonator and a duplexer using the same.
2. Description of Related Art
Development of radio communication and mobile communication technologies requires that components of communication equipment have smaller sizes, higher performance, and lower prices. Specifically, band-pass filters need to have low insertion/reflection loss and high frequency selectivity. However, in the UHF band of 880-960 MHz, long wavelengths of low frequencies make it difficult to makes equipment compact. Therefore, in order to make equipment small while ensuring low insertion/reflection loss and high frequency selectivity, technology for manufacturing Composite Right/Left-Handed (CRLH) filters, as well as duplexer-type filters having a plurality of band-pass characteristics.
FIG. 1A schematically illustrates a conventional duplexer circuit. FIG. 1B illustrates a duplexer device consisting of UHF two-channel local devices. Use of such low-order band-pass filters and local devices as illustrated in FIG. 1 decreases the process cost, but results in poor skirt characteristics and low inter-band isolation.
FIG. 2 illustrates frequency response characteristics of a duplexer using high-order (at least fourth order) band-pass filters. In the drawing, S11, S21, and S31 refer to S-parameters in frequency domain. Specifically, S11 refers to a reflection coefficient, S21 refers to a transmission coefficient of a low-pass filter of the duplexer, and S31 refers to a transmission coefficient of a high-pass filter of the duplexer. FIG. 2 shows that use of at least fourth-order band-pass filters in the range of a number of GHz to design a duplexer improves skirt characteristics of respective bands and isolation between bands. However, such design requires use of plane-stacked half-wavelength resonators, which increase the physical size.
FIG. 3 illustrates a high-order band-pass filter design circuit implemented in a ceramic structure. Such use of a ceramic resonator for a high-order band-pass filter increases the process cost and the product size.
Therefore, there is a need for technology for implementing band-pass filters for the UHF band near 900 MHz, which is popular as commercial communication frequency, as well as SIM band near 2.4 GHz, and duplexers coupling them while guaranteeing low process cost and small product size and, above all, excellent skirt characteristics and isolation.